The apps show locations for those who have signed onto them and are popular among groups of friends and also in families, where parents frequently put them on their children's phones so they know where they are. Friends use them to find each other and keep in touch.

"I used Snap Map to see if my friends were OK," said Aubrey Stanchak, 20, a junior at Pepperdine University in Malibu. Students at the school often frequent the Borderline Bar and Grill, where the shooting unfolded late Wednesday night.

Stanchak checked to make sure her friends who lived near the shooting were at their houses in the middle of the night, and finding them there gave her comfort.

"Knowing they hadn't been active on Snapchat and they were in a place and they'd been there for a while made me feel like, 'OK, they're settled, nothing's happened,'" she said.

In the case of the Thousand Oaks shooting, some families were watching the apps with a sense of grim foreboding as their loved ones appeared to still be checked in at the Borderline Bar and Grill, where Thursday morning only bodies remained.

“The upside is it’s more information than you would otherwise have, which in those situations can be either reassuring or devastating,” said Sierra Filucci, executive editor of parenting content at Common Sense Media, a non-profit that promotes safe technology and media for children.

Adam Housley told the Los Angeles Times that his niece’s Apple Watch and iPhone were showing her location as being on the dance floor at the bar in Thousand Oaks.

Father Jason Coffman told CNN that his son's tracking app on his phone confirmed he was at the club.

"It's there, it's not moving, that’s the problem," he told the network.

Apps such as Life360, FindMyFriends Snapchat's Snap Map and others allow those who've signed on to make themselves trackable.

Life360, for example, saw a spike in app activity in Thousand Oaks starting at 2:00 am Thursday and by 4:00 am was seeing twice the normal number of app sessions in the area, said spokeswoman Sara Sutyak.

They are popular, though controversial, among parents. Some call them "stalking apps" and say they detract from trust within families. Others consider them a way to ensure children are safe in an unsafe world.

In San Francisco, mom Carrie Wonzer uses Find My IPhone to keep track of her teens.

"They know I use it and are sometimes annoyed but they have no expectation of privacy at 15 years old. I have found this to be helpful when they are on the bus and I am trying to pick them up – I look up their location and head for the next bus stop and tell them to get off the bus," she said.

Enabling this kind of tracking is often a struggle for parents who grew up in a time when they had much more independence, away from the eyes of their parents, and comes with both pros and cons.

“For me as a parent, it helps me feel more comfortable letting my kids explore. That reassurance that I can look on my phone and see that they are where they said they were going, or that they’re on their way back home, allows me to feel more comfortable,” said Filucci.

For the digital generation, FindMyFriends and Snapchat's Snap Map are a way to keep track of where their friends are, to find them to hang out with and generally keep track of them.

Previous generations had to rely on having many conversations and building trust with their children to ensure they stayed safe. Today there’s the option of simply tracking them.

“The important thing,” cautioned Filucci, "is that parents don’t use [these programs] as a substitute for all those important conversations.”

One issue these apps do raise is that of privacy. For example, a group of researchers at Oxford University in England that studies third-party tracking looked into what information Life360 was collecting and sharing when asked by USA TODAY.

As of 2017, Life360 shared data with multiple third parties, including Facebook, Google, three third-party mobile marking firms and two location-based mobile marketing firms.

While this information is mentioned in the company’s privacy policy, it’s fairly likely that "most people who use life360 probably aren't aware that these third parties are embedded in the app and collecting information about them,” said Max Van Kleek, a professor of computer science at Oxford University.

Life360's Sutyak noted that data provided to third parties has all personal characteristics stripped off and that users can opt out of data collection.

For Van Kleek, the tragedy in Thousand Oaks “illustrates the complexities and tradeoffs of privacy and transparency especially during emergencies.”